This invention relates to an apparatus for the treatment of molten metal and, more particularly, for degassing molten metal in a sealed treatment zone.
When many molten metals are used for casting and similar processes they must be subjected to a preliminary treatment to remove unwanted components that may adversely affect the physical or chemical properties of the resulting cast product. For example, molten aluminum and aluminum alloys derived from alumina reduction cells or metal holding furnaces usually contain dissolved hydrogen, solid non-metallic inclusions and various reactive elements. The dissolved hydrogen comes out of solution as the metal cools and forms unwanted porosity in the product. Non-metallic solid inclusions reduce metal cleanliness and the reactive elements and inclusions create unwanted metal characteristics. This treatment is frequently carried out in a vessel or trough section in-line with the metallurgical trough used to convey the metal from the holding furnace to the casting machine.
These undesirable components are normally removed from molten metals by introducing a gas below the metal surface by means of gas injectors. As resultant gas bubbles rise through the mass of molten metal, the absorb gases dissolve in the metal and remove them from the melt. This process is often referred to as xe2x80x9cmetal de-gassingxe2x80x9d or xe2x80x9cin-line metal de-gassingxe2x80x9d.
A typical prior gas treatment apparatus for molten metals is described in Waite et al. U.S. Pat. No. 5,660,614, issued Aug. 26, 1997. This apparatus uses a hood to collect off-gases and dust, and various baffles are used to control the flow of metal through the de-gasser. Such a unit does not provide a gas-tight seal.
Molten metals are also treated by means of in-line filtration to remove particulates. Such filters generally consist of box-like or trough-like arrangements containing a filter media which may be a porous refractory plate or granular refractory material in a bed. Filter units are sometimes combined with de-gassers.
There are demands from environmental protection agencies to provide a completely sealed unit for treatment of molten metals so that less particulate is generated and less is exhausted to the atmosphere. The objective is to create a substantially air-free operation which reduces dross and hence dust formation. This is particularly the case for in-line metal degassing where there is a requirement to exhaust the treatment gases from the treatment vessel without leakage into or out of the treatment vessel.
However, equipment for treatment of molten aluminum operates at high temperatures and frequently agitates the molten metal, causing splashing. At the same time operational considerations require periodic opening of the equipment, for example to skim off dross or otherwise clean the equipment.
In English, U.S. Pat. No. 5,846,749, issued Dec. 8, 1998, a metal de-gassing apparatus is described which, along other things, attempts to provide a gas-tight sealed treatment zone. Baffles or underflow weirs are provided at the inlet and outlet of the vessel to act as air-locks, and the patent shows a rather complicated system with a plurality of bolts for holding a cover plate snuggly on the treatment vessel. This does not allow for the fact that the cover must be removed quite often for servicing of gas injectors and for cleaning a residue dross.
In Sarlitto et al., U.S. Pat. No. 5,656,235, issued Aug. 12, 1997, a metal de-gassing apparatus is described which includes air-locks at the inlet and outlet of the vessel. The cover appears in this case to rest on the top surface of the vessel.
It is an object of the present invention to provide an improved form of sealing arrangement between the cover plate of an apparatus for treatment of molten metal and the treatment vessel.
It is a further object of the invention to provide a sealing arrangement which will provide an air-tight seal while permitting the treatment vessel cover to be removed easily.
It is a further object of the invention to provide an in-line metal treatment vessel that provides for minimum gas escape or air inflow whilst being readily opened for dross removal and other servicing requirements.
The present invention in its broadest aspect relates to an apparatus for treating molten metal, preferably as an in-line treatment vessel for treating molten metal flowing between a holding furnace and casting machine. It comprises a treatment vessel for holding molten metal. A removable cover or hood is provided for the treatment vessel with sealing means between the cover and the vessel to provide a gas tight treatment zone. The sealing means comprises mating peripheral flanges on the vessel and cover. Lying between the flanges and adapted to contact the mating faces of both flanges when the cover or hood is in place are at least two strips. These strips include an annular thermal barrier strip formed of a deformable refractory material and adapted to provide a gas leak resistant, thermal barrier between the interior of the vessel and the exterior and an metal strip adapted to provide a mechanical contact between the two mating faces and preferably to control the degree of compression applied to the thermal barrier strip. The at least two strips include an inner metallic face providing a metal splash shield to protect the sealing strips and the space between the mating faces from metal splashes.
The term xe2x80x9cinsidexe2x80x9d as used herein means on the side closer the interior of the treatment vessel and closer to the molten metal held therein. The term xe2x80x9coutsidexe2x80x9d means on the side further from the vessel interior.
The xe2x80x9cremovablexe2x80x9d cover or hood means a hood that can be either completely removed or opened, for example, on hinges attached to the treatment vessel, to permit easy or rapid access to the interior of the treatment vessel.
In one preferred embodiment, the metal strip is located inside the thermal barrier strip and its inner surface acts as the splash protection as well as providing a mechanical contact and spacing control.
In another preferred embodiment, the metal strip is located outside the thermal barrier, that is, the splash protection is separate from the metal strip and is provided as a separate metal strip attached to one of the mating faces. It preferably extends to a point close to, but not necessarily in contact with the opposite mating face so that the outer metal annular strip controls the spacing between the mating faces.
In yet another preferred embodiment, a additional gas sealing strip formed from an elastomeric material is also used, such a seal being in the form of an annular strip located outside the thermal barrier strip, and lying between the mating faces and is thereby thermally protected by the refractory material since such materials are not resistant to either metal splashes or high temperature. More preferably, when the metal strip is separate and outside the thermal barrier strip, the gas sealing strip is located outside the metal strip. The additional gas sealing strip may be used in circumstances where the apparatus must be particularly well sealed. When a gas sealing strip is provided, the metal strip is preferably used to control compression on both the thermal barrier strip and the gas sealing strip.
In trying to obtain a good seal between a cover and a processing vessel, flexible materials are advantageous because of their sealing properties. However, such materials are not generally resistant to the reactive nature of metals being processed. The metals being agitated in the vessel will occasionally splash and contact a seal. Seals having good leak tightness and good thermal resistance are sometimes formed from refractory rope, paper or similar materials, but these have poor resistance to molten aluminum or similar reactive metals, as well as lacking sufficient sealing properties to provide an airtight seal.
In the multi-part seal of the present invention an innermost splash barrier is provided as a first line metal barrier preventing molten metal from penetrating further between the flanges. In some embodiments it also serves to support the flange of the cover plate at a fixed distance above the flange of the treatment vessel and thereby control the degree of compression on the other two sealing strips. The thermal barrier strip is preferably formed from strip of deformable refractory material, preferably in the form of rope, felt or paper, and is of such a size that it is squeezed somewhat between the cover plate and vessel flanges and this provides a partial seal as well as a thermal barrier. Such sealing strips may be formed from an inner core of metal rope or mesh, or of refractory rope, which is then wrapped in a refractory cloth or strip. The metal strip and any splash barrier separate from the metal strip typically have a square or rectangular cross-section, while the thermal barrier strip is typically of a round or oval configuration. The gas sealing strip, when used, may be of square or rectangular cross-section or of a rounded cross-section, e.g. oval or round.
All strips (metal strip, thermal barrier strip, gas sealing strip and metal splash shield) are preferably annular strips that are continuous around the sides and ends of the vessel. This is particularly important for the strip forming the main leak resistant or airtight seal, whether it is the fibrous refractory seal or the optional elastomeric sealing strip. The vessel cover may be round, square, rectangular, etc., and the annular sealing strips generally correspond to the shape of the cover.
In one embodiment of the invention, the flange face on the treatment vessel is flat and the various strips are connected to the flange of the cover.
In another embodiment of the invention, the flange face on the treatment vessel has one or more raised, curved ridges located so that that one or more of the strips will contact the ridges. In particular it is advantageous to provide such a ridge for a gas sealing strip, when used, so as to improve the sealing capabilities.
The cover of the treatment vessel equipped with such multi-part seals can be adapted so that in use it rests on the mating surface of the treatment vessel and seals under its own weight. If more positive sealing is required, quick release clamps may be used, but in either case, sealing is obtained without the use of bolts or similar devices, and the cover can be easily and quickly removed and replaced for servicing.
The treatment vessel of the invention is typically an apparatus for de-gassing molten metal and it may include gas injector rotors extending downwardly from the cover and into the treatment zone. It also typically includes ventilators for safely removing gases from the treatment zone.
The treatment vessel also preferably has baffles, airlocks, or underflow weirs positioned at the entrance and exit of the treatment vessel to provide, along with the seal between the cover and vessel flanges, a completely air-tight and leak-tight vessel for treatment of molten metal.